1. Field of the Invention
The present invention generally relates to a color cathode ray tube assembly and, more particularly, to the color cathode ray tube assembly including a color cathode ray tube provided with a magnetic shield for shielding the tube from an external magnetic field.
2. Description of the Prior Art
In general, it is not unusual that, during the operation of a color cathode ray tube used in, for example, a television receiver set, some of the electron beams travelling within the envelope of the cathode ray tube from an electron gun assembly towards a phosphor deposited screen are adversely affected by unwanted external magnetic fields other than the useful magnetic field developed by the deflection yoke. These unwanted external magnetic fields include those originating from the terrestrial magnetism and/or from various electronic circuit components used in the television receiver set. Once the electron beams are affected by the unwanted external magnetic fields, the electron beams travel having been deviated from their normal courses of travel and finally excite other phosphor dots on the screen than the phosphor dots which they were intended to excite. The erroneous deflection of the electron beams, or some of them, attributable to the unwanted external magnetic fields results in undesirable effects such as a reduction of the color purity.
In order to protect the color cathode ray tube from the unwanted external magnetic fields, various attempts have hitherto been proposed, which include the use of an outer magnetic shield made of an iron plate or a silicon-containing steel plate and surrounding exteriorly of a funnel section of the envelope, the use of an inner magnetic shield made of an iron plate or a silicon-containing steel plate and disposed within the funnel section for improving the magnetic shielding effect, or a combination of both to maximize the magnetic shielding effect. See the U.S. patent application Ser. No. 833,582 filed Feb. 26, 1986, by Kazuhiro Chihara et al., which application corresponds to the Japanese Laid-open Patent Publication No. 62-085592 published Apr. 20, 1987, and the Japanese Laid-open Utility Model Publications No. 62-064084 and No. 61-156166 published Apr. 21, 1987, and Sept. 27, 1986, respectively.
For the purpose of discussion of the prior art believed to be pertinent to the present invention, the color cathode ray tube employing both of the outer and inner magnetic shield will now be described in details with reference to FIGS. 1 and 2 of the accompanying drawings.
FIG. 1 illustrates a side view, with a portion cut away, of a prior art color cathode ray tube and FIG. 2 illustrates an exploded view of the color cathode ray tube shown in FIG. 1. As shown, the color cathode ray tube comprises a highly evacuated envelope 1 including a generally conical funnel section 1b having a large-sized end closed by a generally rectangular faceplate 1a, bonded thereto by the use of glass frit, and a small-sized end integrally continued to a generally cylindrical neck section 1c, and an electron gun assembly (not shown) housed within the neck section 1c at one end thereof opposite to the funnel section 1b. The faceplate 1a has a side flange and a screen plate, said screen plate having an inner surface deposited with a predetermined pattern of primary color elemental phosphor deposits, for example, triads of red, blue and green phosphor dots, thereby to form a phosphor deposited screen 9.
The cathode ray tube also comprises a color selection electrode or apertured shadow mask 2 which is a perforated thin metal foil having a predetermined pattern of apertures which are, in the illustrated example, slots 10. This apertured shadow mask 2 is supported under tension with its peripheral edge secured to a frame structure 3 made of a magnetizable metal of 1 to 2 millimeters in wall thickness. The frame structure 3 carrying the shadow mask 2 in the manner described above has a plurality of generally elongated elastic members 4 rigidly secured at one end to the frame structure 3 so as to extend generally radially outwardly therefrom and at the opposite end engaged to a respective anchor pin (not shown) embedded in the side flange of the faceplate 1a. With the elastic members so engaged to the anchor pins, the frame structure 3 having the apertured shadow mask 2 mounted thereon is supported in position within the envelope 1 with the apertured shadow mask 2 immovably spaced a predetermined distance from the luminescent phosphor deposited screen 9 having a space 20 therebetween.
The internal magnetic shield is generally identified by 5 and is of a shape generally similar to the funnel section 1b and is large enough to exteriorly encircle the electron beams 8 deflected and travailing towards the luminescent phosphor deposited screen 9. This internal magnetic shield 5 is made of a magnetizable metal plate of about 0.1 to about 0.2 millimeters in wall thickness and is retained at a front end by the frame structure 3 while extending from the frame structure 3 towards the boundary between the funnel section 1b and the neck section 1c and generally along the inner wall surface of the funnel section 1b.
The outer magnetic shield is generally identified by 7 and is made of a magnetizable metal plate and is so shaped and so structured as to exteriorly cover the funnel section 1b of the envelope 1. The outer magnetic shield 7 exteriorly covering the funnel section 1b has a generally tubular front extension 7a which exteriorly covers the faceplate 1a and, at the same time, protruding a certain distance frontwardly from the faceplate 1a.
When the color cathode ray tube of the above described construction is operated, the electron beams 8 emitted from the electron gun assembly within the neck section 1c travel through the slots 10 and subsequently impinge upon the phosphor deposited screen 9 to excite the luminescent phosphor dots deposited thereon. Hereinafter, with reference to FIG. 3, description will be made as to how the point of impingement of the electron beams 8 on the phosphor deposited screen 9 will be affected, that is, deflected, by the presence of the terrestrial magnetism when the color cathode ray tube is installed while oriented in a direction conforming to the east direction E, the west direction W, the south direction S and the north direction N.
Referring now to FIG. 3 illustrating the phosphor deposited screen 9 as viewed in a direction from the neck section 1c of the envelope 1, points a to i represent the respective exemplary landing points on the phosphor deposited screen 9 where the electron beams 8 ought to have impinged if they are not affected by the terrestrial magnetism. An arrow-headed vector line drawn from each landing point a to i and designated by one of E, W, S and N represents the direction in which the electron beams 8 intended to impinge upon such landing point a to i is deviated under the influence of the terrestrial magnetism, the length of which vector line represents the magnitude of such deviation. As is well known to those skilled in the art, on the earth there is the terrestrial magnetic field developing from the N-pole in the Antarctic to the S-pole in the Arctic.
Because of the presence of the terrestrial magnetism, as shown in FIG. 3, the direction of the terrestrial magnetic field relative to the phosphor deposited screen 9 extends in the arrow-headed direction E (towards a right-hand portion of the figure of FIG. 3) when the color cathode ray tube is oriented eastwards; in the arrow-headed direction W (towards a left-hand portion of the figure of FIG. 3) when the color cathode ray tube is oriented westwards; in the arrow-headed direction S (in one direction rearwardly of and perpendicular to the figure of FIG. 3) when the color cathode ray tube is oriented southwards; and in the direction N (in a direction forwardly of and perpendicular to the figure of FIG. 3) when the color cathode ray tube is oriented northwards. The electron beams 8 are generated from the electron gun assembly which is disposed inside the neck section 1c of the envelope 1 in alignment with the geometric center of the phosphor deposited screen 9 represented by the landing point e. The electron beams 8 so generated are, during their travel towards the phosphor deposited screen 9, deflected under the influence of the magnetic field developed by the deflection yoke so as to scan the phosphor deposited screen 9 horizontally and vertically from top to bottom of the phosphor deposited screen 9. Accordingly, as Fleming's left-hand rule makes it clear, the electron beams 8 ready to impinge upon the phosphor deposited screen 9 shift in such respective vector directions as shown in FIG. 3. In other words, where the color cathode ray tube are installed oriented eastwards or westwards, the electron beams 8 shift upwardly or downwardly of the phosphor deposited screen 9, but where the color cathode ray tube are installed oriented southwards or northwards, the electron beams 8 shift in a direction tangential to the circle depicted about the landing point e and in such respective direction as shown in FIG. 3.
FIG. 4 illustrates, on an enlarged scale, a portion of the phosphor deposited screen 9 on which triads of finely spaced and alternating stripes of primary color elemental phosphor deposits, for example, blue, red and green phosphor dots, designated by B, R and G, respectively. Each row of the slots 10 defined in the apertured shadow mask 2 corresponds in position to the respective triad of stripes B, R and G of primary color elemental phosphor deposits on the screen 9 and, therefore, the electron beams generated from the electron gun assembly when they impinge upon the corresponding phosphor stripes B, R or G after having passed through the slots 10 in the apertured shadow mask 2 excite such phosphor stripes permitting them to emit luminescent light.
In the case of the color cathode ray tube of the type wherein the luminescent phosphor deposits are formed in stripes such as shown in and described with reference to FIG. 4, the electron beam of a certain color necessarily impinges upon the phosphor stripe of the same color even though such electron beam is deviated upwardly or downwardly under the influence of the terrestrial magnetism, and, therefore, no reduction in color purity occurs substantially.
However, since the outer magnetic shield 7 used in the prior art color cathode ray tube and shown in FIG. 2 is of a generally rectangular shape, it has been found that, when the color cathode ray tube is installed oriented eastwards or westwards, magnetic fluxes tend to be disturbed at four corner areas of the outer magnetic shield 7. FIG. 5 illustrates the extent to which the magnetic flux is disturbed at one of the four corner areas of the outer magnetic shield 7 when the color cathode ray tube is oriented eastwards or westwards. As FIG. 5 makes it clear, where the color cathode ray tube is installed while oriented eastwards or westwards and is exposed to the magnetic field developed by the terrestrial magnetism, the magnetic fluxes develop in a direction leftwards and rightwards as viewed in FIG. 5. In such case, the magnetic fluxes 11 concentrated at a top or bottom portion of each shorter side wall 7ab of the shield extension 7a are discharged as magnetic leakage fluxes 12a towards a right-hand or left-hand portion of the adjacent longer side wall 7aa of the front extension 7a and then flow towards the associated longer side wall 7aa of the outer magnetic shield 7 where the magnetic resistance is low. However, since no shield is disposed within the space 20 delimited between the phosphor deposited screen 9 and the apertured shadow mask 2 as shown in FIG. 1, the magnetic leakage fluxes 12a so discharged affect the passage of the electron beams 8 then travelling in the vicinity of each corner area of the extension 7a of the outer magnetic shield 7 to such an extent as to result in the reduction in color purity.
Also, the outer magnetic shield 7 shown in FIG. 2 and used in the prior art color cathode ray tube has a plurality of, for example, two, cutouts 7e defined in each longer side wall 7aa of the shield extension 7a thereof for the passage therethrough of corresponding fixtures 30 that are used to secure the color cathode ray tube to a support structure fast or integral with the chassis in the television receiver set. Therefore, as shown in FIG. 5, different magnetic leakage fluxes 12b are produced at each cutout 7e in the longer side walls 7aa of the shield extension 7a, which fluxes 12b may also bring about adverse influence on the electron beams 8 travailing through the apertured shadow mask 2 towards the phosphor deposited screen 9 in a manner substantially similar to the magnetic leakage fluxes 12a, thereby to constitute an additional cause of the reduced color purity.
It is a recent trend that the color cathode ray tube are increasing in size. The larger the size of the color cathode ray tube, the larger the space 20 between the apertured shadow mask 2 and the phosphor deposited screen 9. For example, in the case of 37-inch, 110.degree. deflection color cathode ray tube having the generally rectangular faceplate 9, the space 20 amounts to about 50 millimeters, and the amount of shift of the electron beams in the horizontal direction at the landing points a, c, g and i shown in FIG. 3 will be 148 micrometers when the magnetic field in the east-to-west direction is changed by 0.8 gausses.
Although the critical amount of shift of the electron beams varies depending on the type of the color cathode ray tube, the value of the critical shift amount (150 micrometers) is considered a standard value by which the color cathode ray tube can be determined practically acceptable in the case of 37-inch, 110.degree. deflection color cathode ray tube.
As described above, the amount of shift of the electron beams in the prior art color cathode ray tube is of a value approximately equal to the critical shift amount of 150 micrometers and, in order to minimize the possible reduction in color purity due to the increased amount of shift of the electron beams resulting from variation in the space 20 during the manufacture of the color cathode ray tubes, the average value of the amounts of shift of the electron beams has to be minimized.
Also, in order to increase the effectiveness of counteracting, or canceling, the external magnetic field, an attempt has been as shown in FIG. 6 proposed wherein a N/S canceler coil 13 is disposed inside the front extension 7a of the outer magnetic shield 7 so as to extend along the longer side walls 7aa and the shorter side walls 7ab, it being, however, to be noted that, in FIG. 6, Bv and E/W canceler coils are not illustrated for the purpose of simplicity. It is to be noted that Bv, N/S and E/W herein referred to represent vertical component, north-to-south component and east-to-west component of the external magnetic field, respectively. The outer magnetic shield 7 of the construction shown in FIG. 6 can be used in combination with the color cathode ray tube having the inner magnetic shield such as shown in FIG. 1 or with the color cathode ray tube having no inner magnetic shield. Since the canceler coil 13 is covered by the outer magnetic shield 7, the magnetic field developed by the flow of an electric current through the canceler coil 13 can effectively act on the color cathode ray tube with the outer magnetic shield 7 acting as a magnetic core, thereby increasing the canceler efficiency, that is, the efficiency of counteracting or canceling the external magnetic field.
However, according to the canceling system shown in and described with reference to FIG. 6, it has been found difficult to make a pattern of distribution of the magnetic field developed by the canceler coil 13 match with that of the external magnetic field such as the terrestrial magnetic field and, therefore, the occurrence of a shift of the electron beams cannot be eliminated at any point on the phosphor deposited screen. To describe in more detail, while the electron beams traveling towards the phosphor deposited screen through the apertured shadow mask tend to be deviated from their normal courses of travel under the influence of the external magnetic field, such as, for example, the terrestrial magnetic field, and finally impinge upon other phosphor dots on the screen than the phosphor dots which they were intended to impinge upon, it has been found difficult to effectively eliminate such deviation of the electron beams even when the electric current of an appropriate value is allowed to flow through the canceler coil 13.
In the known arrangement shown in FIG. 7, the canceler coil 13 is mounted exteriorly on the front extension 7a of the outer magnetic shield 7 except for portions 13a and 13b thereof extending inwardly of the respective longer side walls 7aa. More specifically, the canceler coil 13 shown in FIG. 7 have each portions 13a and 13b extending along an inner surface of a portion of the associated longer side wall 7aa delimited between the cutouts 7e while the remaining portion thereof extends exteriorly along the outer surfaces of the shorter side walls 7b. The cutouts 7e are provided for the passage of the fixtures 30 rigid with the cathode ray tube as is the case with those shown in FIG. 2. This is disclosed in, for example, the U.S. patent application Ser. No. 864,659 filed May 19, 1986, by Kazuhiro Chihara et al., which application corresponds to the Japanese Laid-open Patent Publications No. 61-264992 published Nov. 22, 1986, and No. 61-289787 published Dec. 19, 1986.
According to the system shown in FIG. 7, where the effect of canceling the external magnetic field is desired to be enhanced, the canceler coil 13 are allowed to extend inside the outer magnetic shield 7 and along the inner surfaces of the respective portions of the longer side walls 7aa delimited between the cutouts 7e as indicated by that portions 13a and 13b whereas, where the canceling effect is desired to be weakened, the canceler coil 13 is allowed to extend exteriorly of the outer magnetic shield 7 and along the outer surfaces of the respective shorter side walls 7ab as indicated by portions 13c and 13d. By so doing, a desired pattern of distribution of the magnetic field counteracting or canceling the external magnetic field can be obtained.
As is the case with the system shown in and described with reference to FIG. 6, in FIG. 7 only the N/S canceler coil 13 for counteracting the external magnetic field acting in a direction generally parallel to the longitudinal axis of the color cathode ray tube is illustrated with the Bv and E/W canceler coils omitted. However, the Bv and E/W canceler coils are not always necessary if each of the side walls 7aa and 7ab has a length sufficient to overhang frontwardly of the faceplate 1a.
As hereinabove discussed, in the system of FIG. 7, each of the cutouts 7e defined in the longer side walls 7aa of the outer magnetic shield 7 is cut from the front edge thereof inwardly towards a direction generally parallel to the longitudinal axis of the color cathode ray tube 1. The canceler coil 13 have each portions 13a and 13b extending along an inner surface of a portion of the associated longer side wall 7aa delimited between the cutouts 7e while the remaining portions 13c and 13d thereof extends exteriorly along the outer surfaces of the shorter side walls 7ab.
As is well known to those skilled in the art, when the color cathode ray tube is installed while oriented northwards where the terrestrial magnetism exists, the electron beams impinging upon the phosphor deposited screen 9 undergoes a rotational shift as shown by the arrow-headed lines in FIG. 8, constituting a cause of color misconvergence and a raster rotation. Although the extent to which the electron beams shift varies depending on the angle of deflection of the color cathode ray tube 1, the size of the faceplate thereof, the specific structure of the electron gun assembly and other factors, the amount of shift varies from point to point on the phosphor deposited screen 9. By way of example, when the amount of deviation of the electron beams impinging upon the phosphor deposited screen 9, that is, the amount of misalignment of the electron beams, at each of locations shown by T, TTR, TR, TTR, and R in FIG. 8 are plotted on a graph corresponding to the top right-hand square area of the screen of the color cathode ray tube, such a curve shown by a in FIG. 9 can be obtained. The curve a shown in FIG. 9 has been obtained by plotting the amounts (expressed in terms of micrometer) of misalignment of the electron beams in a horizontal direction after having been demagnetized, which misalignment has occurred in the 37-inch, 110.degree. deflection color cathode ray tube having the stripe-patterned phosphor deposited screen and also having the outer magnetic shield 7. During the test which has resulted in the curve a, a terrestrial magnetic force of about 0.3 gauss is applied to the color cathode ray tube in a direction generally parallel to the longitudinal axis of the color cathode ray tube and then the above mentioned demagnetization was carried out.
In the graph of FIG. 9, a curve b1, illustrates the amount of canceling compensation exhibited by the system of FIG. 6, and a curve b2 represents the amount of shortage of compensation. In other words, when the electric current sufficient to eliminate the misalignment of the electron beams at the point TR in one corner area of the phosphor deposited screen 9, that is, sufficient to render the amount of deviation of the electron beams aimed at the corner area TR to be zero, is supplied through the canceler coil 13 shown in FIG. 6, a sufficient compensation for the deviation of the electron beams cannot be achieved at the points T and TTR on the phosphor deposited screen 9 whereas the deviation of the electron beams at the point TRR is excessively compensated for. This is evidently shown by the curve b2 in the graph of FIG. 9. Note that the curve b1 is generally opposite in polarity to that exhibited by the curve a.
According to the system shown in and described with reference to FIG. 7, the amount of canceling compensation is exhibited by a curve c1 and the amount of shortage of compensation is exhibited by c2. Comparing the curve c2 exhibited by the system of FIG. 7 with the curve b2 exhibited by the system of FIG. 6 makes it clear that the amount of shortage of compensation in the system of FIG. 7 is considerably reduced. The reason for thin difference is discussed hereinafter.
Where the canceler coil 13 is positioned inside the outer magnetic shield 7 with the outer magnetic shield 7 serving as a magnetic core, that is, where the canceler coil 13 is positioned on one side of the outer magnetic shield 7 closer to the color cathode ray tube 1, a canceling magnetic field of relatively high density is developed inside the outer magnetic shield 7. On the other hand, where the canceler coil 13 is disposed outside the outer magnetic shield 7, such a canceling magnetic field of relatively high density is developed outside the outer magnetic shield 7. Hence, where the canceler coil 13 is disposed outside the outer magnetic shield 7, the resultant canceling magnetic field does not act on the color cathode ray tube so much as intended.
Even in the system of FIG. 7, when the canceler coil 13 is disposed in part inside the outer magnetic shield 7 and in part outside the outer magnetic shield 7, an unbalanced condition in which the misalignment of the electron beams at the point TTR is insufficiently compensated for and that at the points TR and TRR is excessively compensated for is still observable and, therefore, the system of FIG. 7 is far from the effective method of minimizing the misalignment of the electron beams at any point on the phosphor deposited screen. This appears attributable to the provision of the cutouts 7e at which the canceling effect tends to be reduced.
Unlike the use of the combination of the outer magnetic shield with the canceler coils shown in and described with reference to FIG. 7, RCA's A51-161X model color cathode ray tube now commercially available makes use of series-connected canceler (degaussing) coils mounted exteriorly on the funnel section of the envelope through a mounting strap so as to cover the external coating formed on the outer surface area of the funnel section. According to this model, the assembly of series-connected canceler coils does not extend over the circumference of the envelope, particularly the funnel section of the envelope, and, therefore, the misalignment of the electron beams can not be completely eliminated at any point on the phosphor deposited screen.